Nanotechnology-Enabled Sensors

3.5.3 MOS Capacitor based Transducers
3.5 Solid State Transducers 111
The metal oxide semiconductor (MOS) capacitor consists of a metal deposited
over a thin oxide layer, which is in turn deposited over a semiconductor.
20 A typical example of a MOS capacitor based transducer can be
seen in Fig. 3.34. The oxide layer is typically a native oxide of the semiconductor
(e.g. SiO2 on Si). When a voltage is applied across it, the device
appears like a parallel plate capacitor. The dielectric properties of the
oxide and semiconductor change when the device is exposed to different
environmental conditions. Capacitance is a function of the dielectric properties,
and hence capacitance-voltage (C-V) characteristics of such devices
are generally obtained during a sensing experiment.
A time varying AC signal (Fig. 3.34) is applied to the MOS device and
the impedance is measured, from which the capacitance is obtained.
Power
Supply
Impedance meter
Metal
Oxide
Semiconductor
Fig. 3.34 Example of a MOS capacitor based transducer system.
Z
When positive or negative voltages are applied to MOS capacitors, three
regimes may exist on the semiconductor surface. These are inversion, depletion
and accumulation. Fig. 3.35 shows these regimes for an n-type
semiconductor. Applying a positive voltage to the metal attracts electrons
(majority carriers for n-type semiconductors) from the substrate to the oxide-semiconductor
interface, causing them to accumulate there. The top of
the conduction band bends downwards and current flows through the
structure. Applying a small negative voltage causes the bands to bend upwards,
repelling electrons until they are depleted from the interface. Decreasing
the voltage further causes holes (minority carriers for n-type
semiconductors) to accumulate at the interface, and the device is said to be
in the inverted regime. 20